Nampa, ID rainwater: 8,751 gallons off a roof
Rainwater from a roof in Nampa, ID
8,751gallons a year
That is a 1,500 sq ft asphalt roof under Nampa, ID's normal 11.0 inches of rain, about 795 gallons for every inch that falls.
Nampa, ID's rain arrives in a wet season and stops, so storage is what matters. It takes a cistern of 1,000 gallons to carry spring rain into the dry months.
Rainfall: NOAA 1991-2020 climate normals, the Nampa Sugar Factory station (1.3 mi away). Nampa, ID ranks 22nd wettest of the 26 cities covered in Idaho.
Month by month in Nampa
| Month | Rain (in) | Off the roof (gal) | Garden shortfall (gal) |
|---|---|---|---|
| Jan | 1.37 | 1,089 | |
| Feb | 1.04 | 827 | |
| Mar | 1.26 | 1,001 | |
| Apr | 1.06 | 842 | 410 |
| May | 1.37 | 1,089 | 372 |
| Jun | 0.73 | 580 | 451 |
| Jul | 0.26 | 207 | 510 |
| Aug | 0.20 | 159 | 517 |
| Sep | 0.36 | 286 | 497 |
| Oct | 0.80 | 636 | 443 |
| Nov | 1.17 | 930 | |
| Dec | 1.39 | 1,105 |
Dec is the wettest month (1.39 in, 1,105 gal off the roof); Aug the driest (0.20 in). Garden shortfall is what a 200 sq ft vegetable bed needs beyond the rain that lands on it, April to October.
What size rain barrel for Nampa
Best fit: a 1,000-gallon cistern, which delivers about 2,930 gallons to the garden in a normal year (91.6% of its need).
If you set one up, ask about a rebate first: cistern-sized storage costs real money, and a buried or tall tank may need a permit. Place it where a full tank (about 8.3 lb a gallon) sits on firm, level ground.
| Tank | Water used (gal/yr) | Spills over (gal/yr) |
|---|---|---|
| 55 gal | 921 | 7,260 |
| 110 gal | 1,595 | 6,586 |
| 165 gal | 1,917 | 6,264 |
| 275 gal | 2,205 | 5,976 |
| 550 gal | 2,480 | 5,701 |
| 1,000 gal | 2,930 | 5,251 |
| 1,500 gal | 2,975 | 5,206 |
| 2,500 gal | 2,975 | 5,206 |
| 5,000 gal | 2,975 | 5,206 |
Simulated storm by storm through a normal year using the station's count of days with 0.1 in or more of rain each month.
Is rainwater collection legal in Idaho?
Allowed through a legal clarification made by the Office of the Attorney General
From the 2015 (Pacific Northwest National Laboratory report 24347); not yet re-checked against current law, so check with your state water agency before building anything large. Source